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Blood Pressure

Blood moves through the arteries, arterioles, and capillaries because of the force created by the contraction of the ventricles.

Blood pressure in the arteries.

The surge of blood that occurs at each contraction is transmitted through the elastic walls of the entire arterial system where it can be detected as the pulse. Even during the brief interval when the heart is relaxed — called diastole — there is still pressure in the arteries. When the heart contracts — called systole — the pressure increases.

Blood pressure is expressed as two numbers, e.g., 120/80.

Blood pressure in the capillaries

The pressure of arterial blood is largely dissipated when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7.5 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total cross-sectional area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases.

Blood pressure in the veins

When blood leaves the capillaries and enters the venules and veins, little pressure remains to force it along. Blood in the veins below the heart is helped back up to the heart by the muscle pump. This is simply the squeezing effect of contracting muscles on the veins running through them. One-way flow to the heart is achieved by valves within the veins

Exchanges Between Blood and Cells

With rare exceptions, our blood does not come into direct contact with the cells it nourishes. As blood enters the capillaries surrounding a tissue space, a large fraction of it is filtered into the tissue space. It is this interstitial or extracellular fluid (ECF) that brings to cells all of their requirements and takes away their products. The number and distribution of capillaries is such that probably no cell is ever farther away than 50 µm from a capillary.

When blood enters the arteriole end of a capillary, it is still under pressure produced by the contraction of the ventricle. As a result of this pressure, a substantial amount of water and some plasma proteins filter through the walls of the capillaries into the tissue space.

Thus fluid, called interstitial fluid, is simply blood plasma minus most of the proteins. (It has the same composition and is formed in the same way as the nephric filtrate in kidneys.)

Interstitial fluid bathes the cells in the tissue space and substances in it can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid.

Near the venous end of a capillary, the blood pressure is greatly reduced .Here another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and thus a somewhat greater concentration of water. This difference sets up an osmotic pressure. Although the osmotic pressure is small, it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid reenters the capillary here.

Control of the Capillary Beds

An adult human has been estimated to have some 60,000 miles of capillaries with a total surface area of some 800–1000 m². The total volume of this system is roughly 5 liters, the same as the total volume of blood. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. So a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. During vigorous exercise, for example, capillary beds in the skeletal muscles open at the expense of those in the viscera. The reverse occurs after a heavy meal.

The walls of arterioles are encased in smooth muscle. Constriction of arterioles decreases blood flow into the capillary beds they supply while dilation has the opposite effect. In time of danger or other stress, for example, the arterioles supplying the skeletal muscles will be dilated while the bore of those supplying the digestive organs will decrease. These actions are carried out by

the autonomic nervous system.
local controls in the capillary beds

Plasma: is the straw-colored liquid in which the blood cells are suspended.

Composition of blood plasma
Component	Percent
Water	~92
Proteins	6–8
Salts	0.8
Lipids	0.6
Glucose (blood sugar)	0.1

Plasma transports materials needed by cells and materials that must be removed from cells:

various ions (Na⁺, Ca²⁺, HCO₃⁻, etc.
glucose and traces of other sugars
amino acids
other organic acids
cholesterol and other lipids
hormones
urea and other wastes

Most of these materials are in transit from a place where they are added to the blood

exchange organs like the intestine
depots of materials like the liver

to places where they will be removed from the blood.

every cell
exchange organs like the kidney, and skin.

HEART DISORDERS

Pump failure => Alters pressure (flow) =>alters oxygen carrying capacity.
1. Renin release (Juxtaglomerular cells) Kidney
2. Converts Angiotensinogen => Angiotensin I
3. In lungs Angiotensin I Converted => Angiotensin II
4. Angiotensin II = powerful vasoconstrictor (raises pressure, increases afterload)
  1. stimulates thirst
  2. stimulates adrenal cortex to release Aldosterone
    (Sodium retention, potassium loss)
  3. stimulates kidney directly to reabsorb Sodium
  4. releases ADH from Posterior Pituitary
Myocardial Infarction
1. Myocardial Cells die from lack of Oxygen
2. Adjacent vessels (collateral) dilate to compensate
3. Intracellular Enzymes leak from dying cells (Necrosis)
  1. Creatine Kinase CK (Creatine Phosphokinase) 3 forms
    1. One isoenzyme = exclusively Heart (MB)
    2. CK-MB blood levels found 2-5 hrs, peak in 24 hrs
    3. Lactic Dehydrogenase found 6-10 hours after. points less clearly to infarction
  2. Serum glutamic oxaloacetic transaminase (SGOT)
    1. Found 6 hrs after infarction, peaks 24-48 hrs at 2 to 15 times normal,
    2. SGOT returns to normal after 3-4 days
4. Myocardium weakens = Decreased CO & SV (severe - death)
5. Infarct heal by fibrous repair
6. Hypertrophy of undamaged myocardial cells
  1. Increased contractility to restore normal CO
  2. Improved by exercise program
7. Prognosis
  1. 10% uncomplicated recovery
  2. 20% Suddenly fatal
  3. Rest MI not fatal immediately, 15% will die from related causes
Congenital heart disease (Affect oxygenation of blood)
1. Septal defects
2. Ductus arteriosus
3. Valvular heart disease
  1. Stenosis = cusps, fibrotic & thickened, Sometimes fused, can not open
  2. Regurgitation = cusps, retracted, Do not close, blood moves backwards

The defecation reflex:

As a result of the mass movements, pressure is exerted on the rectum and on the internal anal sphincter, which is smooth muscle, resulting in its involuntary relaxation. Afferent impulses are sent to the brain indicating the need to defecate. The external sphincter is voluntary muscle and is controlled by the voluntary nervous system. This sphincter is relaxed along with contraction of the rectal and abdominal muscles in the defecation reflex

Cardiac Output:

Minute Volume = Heart Rate X Stroke Volume

Heart rate, HR at rest = 65 to 85 bpm

Each heartbeat at rest takes about .8 sec. of which .4 sec. is quiescent period.

Stroke volume, SV at rest = 60 to 70 ml.

Heart can increase both rate and volume with exercise. Rate increase is limited due to necessity of minimum ventricular diastolic period for filling. Upper limit is usually put at about 220 bpm. Maximum heart rate calculations are usually below 200. Target heart rates for anaerobic threshold are about 85 to 95% of maximum.

Terms:

End Diastolic Volume, EDV - the maximum volume of the ventricles achieved at the end of ventricular diastole. This is the amount of blood the heart has available to pump. If this volume increases the cardiac output increases in a healthy heart.

End Systolic Volume, ESV - the minimum volume remaining in the ventricle after its systole. If this volume increases it means less blood has been pumped and the cardiac output is less.

EDV - ESV = SV

SV / EDV = Ejection Fraction The ejection fraction is normally around 50% at rest and will increase during strenuous exercise in a healthy heart. Well trained athletes may have ejection fractions approaching 70% in the most strenuous exercise.

Isovolumetric Contraction Phase - a brief period at the beginning of ventricular systole when all valves are closed and ventricular volume remains constant. Pressure has risen enough in the ventricle to close the AV valves but not enough to open the semilunar valves and cause ejection of blood.

Isovolumetric Relaxation Phase - a brief period at the beginning of ventricular diastole when all valves are closed and ventricular volume is constant. Pressure in the ventricle has lowered producing closure of the semilunar valves but not opening the AV valves to begin pulling blood into the ventricle.

Dicrotic Notch - the small increase in pressure of the aorta or other artery seen when recording a pulse wave. This occurs as blood is briefly pulled back toward the ventricle at the beginning of diastole thus closing the semilunar valves.

Preload - This is the pressure at the end of ventricular diastole, at the beginning of ventricular systole. It is proportional to the End Diastolic Volume (EDV), i.e. as the EDV increases so does the preload of the heart. Factors which increase the preload are: increased total blood volume, increased venous tone and venous return, increased atrial contraction, and the skeletal muscular pump.

Afterload - This is the impedence against which the left ventricle must eject blood, and it is roughly proportional to the End Systolic Volume (ESV). When the peripheral resistance increases so does the ESV and the afterload of the heart.

The importance of these parameters are as a measure of efficiency of the heart, which increases as the difference between preload and afterload increases

Glomerular filtration

Kidneys receive about 20% of cardiac output , this is called Renal Blood Flow (RBF) which is approximatley 1.1 L of blood. Plasma in this flow is about 625 ml . It is called Renal Plasma Flow (RPF) .
About 20 % of Plasma entering the glomerular capillaries is filtered into the Bowman`s capsule .
Glomerular filtration rate is about 125 ml/min ( which means 7.5 L/hr and thus 180 L/day) This means that the kidney filters about 180 liters of plasma every day.

The urine flow is about 1ml/min ( about 1.5 liter /day) This means that kidney reabsorbs about 178.5 liters every day .

Filtration occurs through the filtration unit , which includes :

1- endothelial cells of glomerular capillaries , which are fenestrated . Fenestrae are quite small so they prevent filtration of blood cells and most of plasma proteins .

2- Glomerular basement membrane : contains proteoglycan that is negatively charged and repels the negatively charged plasma proteins that may pass the fenestrae due to their small molecular weight like albumin . so the membrane plays an important role in impairing filtration of albumin .

3- Epithelial cells of Bowman`s capsule that have podocytes , which interdigitate to form slits .

Many forces drive the glomerular filtration , which are :

1- Hydrostatic pressure of the capillary blood , which favours filtration . It is about 55 mmHg .

2- Oncotic pressure of the plasma proteins in the glomerular capillary ( opposes filtration ) . It is about 30 mm Hg .

3- Hydrostatic pressure of the Bowman`s capsule , which also opposes filtration. It is about 15 mmHg .

The net pressure is as follows :

Hydrostatic pressure of glomerular capillaries - ( Oncotic pressure of glomerular capillaries + Hydrostatic pressure of the Bowman capsule):
55-(35+10)
=55-45
=10 mmHg .

Te glomerular filtration rate does not depend only on the net pressure , but also on an other value , known as filtration coefficient ( Kf) . The later depends on the surface area of the glomerular capillaries and the hydraulic conductivity of the glomerular capillaries.

Physiology - science that describes how organisms FUNCTION and survive in continually changing environments

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